1. Field of the Invention
The present invention relates to an imaging device such as a digital camera including a solid-state image sensor to acquire image data of a subject, in particular to an imaging device which performs good imaging control in a bulb mode and an imaging control method for such an imaging device.
2. Description of the Prior Art
In prior art there is a digital camera having a bulb mode in which a photo is taken in a long exposure time while a shutter is kept open (disclosed in Japanese Patent Application Publication No. 2002-27326 (Reference 1), for example). Such a digital camera has the same problem as a silver-salt camera that an operator cannot check a change in exposure amount on a display during the exposure to determine the right shooting timing with an intended exposure level. This requires for the operator to estimate an exposure time from his/her experience and check a reproduced image on a display. In a case that the image is not a successful shot, he/she has to re-take a photo.
In order to deal with such a problem, Japanese Patent Application Publication No. 2005-117395 (Reference 2), No. 2005-354166 (Reference 3), and No. 2009-130470 (Reference 4) disclose imaging devices which allow users to check a change in the exposure amount on a display by repetitively reading image data at a predetermined interval during exposure and adding a previously read frame and a currently read frame to update an image display.
Specifically, Reference 1 discloses a technique to display live preview images using a rolling shutter of a CMOS (complementary metal-oxide semiconductor) sensor. References 2 to 4 disclose imaging devices which display preview images in a bulb mode.
Recently, use of the CMOS sensor in an imaging device as described in Reference 1 has increased owing to a high resolution thereof, replacing a widely used CCD (charge coupled device). With use of the CCD, incident light need be blocked with a mechanical shutter during a transfer period even in the bulb mode. This leads to a problem that data of a subject is missed out during transfer periods which are repeated with a predetermined interval. For example, when capturing a moving subject such as fireworks, a captured image will be discrete trails. With use of another drive system instead of the mechanical shutter for the purpose of avoiding such a problem, a different problem as occurrence of smears arises.
In contrast, the rolling shutter of the CMOS image sensor can read image data with high resolution without transfer periods and it does not generate smears. Although the CMOS image sensor has an intrinsic drawback that an image of a moving subject may be distorted due to non-simultaneous exposure of the rolling shutter and transverse stripes may occur in a single image under flickering lights, these do not cause deterioration in image quality in the bulb mode in which exposure time is expected to be over 1 second.
Any of the above imaging devices cannot provide sufficient usability in bulb shooting and there is a demand for improving preview display in the bulb mode.
In the bulb shooting, exposure amount logarithmically rises relative to exposure time. Because of this, immediately after start of exposure, the exposure amount sharply rises and thereafter it gradually rises. In shooting a subject which takes about several seconds to reach an intended exposure amount, an operator needs to quickly decide the timing to complete the bulb exposure, following the sharp rise of the exposure amount while checking a change in the exposure amount on the display. On the other hand, in shooting a subject which takes over several dozen seconds to reach an intended exposure amount, an operator has to wait for the exposure amount to gradually rise to the intended amount.
There is one way to solve the above problem with the bulb mode that display update interval is set independently from data read interval during exposure. For instance, it is possible to reduce a change in the rising speed of the exposure amount on the display by updating image display with a longer interval when the rise in the exposure amount is sharp and updating it with a shorter interval when the rise in the exposure amount is gradual.